Reduction of fading of radiosignals



y 9, 1929- DE Loss K. MARTIN 7 9.896

REDUCTION OF FADING OF RADIOSIGNALS Filed Feb. 24, 1927 IN V EN TOR.

ATTORNEY titl Patented July 9, 1929.

orn'rso sra'rss 13E LOSS PHQNE AND TELEGRAPH COltIPANY, A COPJPURATIONOF NEW "YORK.

REDUCTION OF FADING 0F Rt-tDIGElIGrNALS.

Application filed February 24, 19557.

This invention relates to improven'ients in methods of and means forreducing the fading of radio signals.

The term fading, as applied to radio signals, used with the commonlyaccepted meaning, referring to the relatively slow changes in theamplitude of the signal with time. The phenomena of fading is usuallyn'mnifested at night, but is not limited to this general. period oftime, as signal fading has also been observed during the sunlit periodof the day. The fading of the radio signal is Cttl'ISOtl by theinterference of two or more signals which are varyin in an'iplitude andphase; the received signal is, of course, the resulltzmt of theii'itcrfering signals. lf adjing also variable with respect to space;this include the physical position of the receiving a tenna and thespace al'isorption characteristic of the antenna. The receiving signal.may be conceived as being the resultant of several. interfering wavefronts which are traversing a n'iedi um which is changeabie in itscharacteristics. no component wave may be assumed to be intimatelyassociated with the conductivity of the earths surface over which itpassing. lhe transmission of this wave component. termed the directwave, would not be modified by the presence or absence of sunlight asthere is no reason to bciicve that light affects the conductivity of theground. However, the other component waves traversing the upperatn'iosphere are materially inodilied by the absence of sunli 'h toselect with suitable means, the con'ipenen t or direct wave, from theresultant which will i've uni form a in pli tude. i

in n'acticc, however, it is very ditlicult to make this selection, as itis necessary to balance out the variable components by combiuing out ofphase these variable signals from two or more receiving points, and dueto the very nature of these variable componcnts it probably would beimpossible to get a perfect balance. Or the direct wave signals may bebalanced out, leaving the combination of variable components whoseresultant also would be expected to be variable to a degree. Hence, acompromise of these two methods would seem to afiord a desirable methodof obtaining a solution. Accordingly, it is the general purpose of thearrange-- rnents of this invention to combine the signals from two ormore receiving points and to t. Hence, in theory, it would be desirableSerial No. 179,624.

Iii. MARTIN, OF WEST ORANGE, NEW JERSEY, ASSIGNOB .110 AMEBJEUA'N TELE-i ven The invention may be more fully understood from the followingdescription, togetl'ier with the accompanying drawing, in. the Figures 1and .2 of which a re shown circuit diagrams embodying one form oi theinvention and a nunlilication thereof. lar reference characters havebeen. utilized to denote like parts in both of the figures of the d rawi 2 lg.

in Fig. l is shown a receiving circuit which takes advantage of thedifference in the space absmption charucteristics of two loop an tennmvL, and. L placed with their axes perpendicular to each other. Each loopis tuned by the condensers U, and to receive signals of the nnefrequency. The received signals from each loop circuit are amplified anddctcctcd separately by the radio frequency are.- plitiers lil A itl Aand the detectors l), and. D The audio signals from. the detectors arecombined by connecting in series the secondaries oil. the detectors and(.uitput transilorluers it, and T whence they nary be ampl i lied by theaudio frequency atupli tier i'tl fl and trmlsmittcd through a trans areceiving circuit.

former T to The signals froih each of the loops l), and la, will havedi'tli'cu'cut fading clunructeristics due to the dircctimial propiu'ticsof the loop antcnruc and the variations in the i "are front of thetransniittcd wave at the receiver. The signal from one loop may beincreasing and the sigi'ial from. the other decreasing, hence bycombining the two some inr 'irovewent in the stability of the signal maybe expccted. But experience with similar receivin circuits where theaudio signals were co inbincd has demonstrated that satisfactoryuniforniity in signal amplitude is not oljltained. Accordingly, toimprove the stability of the signal over that obtained by simpleUOllllJllltl tion, means are provided in the receivin cir cuits shown toregulate automatically the gain of the radio frequency amplifiers inaddition to combining the audio signals at the output of the detectors.

The gain in the radio frequency amplifiers is controlled by eha The gridpcten .i

n g" in are till l. 1 ll varied by applying to the respective gridcircuits the voltage drop along the resistances R and R caused by thevarying plate circuit current of the detectors D and D In the detectorcircuits C and G, are the usual radio frequency by-passing condensers,and C and G are audio frequency by-passing condensers for tuning thecircuit for the purpose of preventing an audio frequency voltage drop inthe resistances R and R The operation of the arrangements is as follows:lhe variation in the plate current of the detectors D and D is primarilydue to the fading of the carrier signal impressed on the grid of thedetector tubes from the output circuit of the radio frequencyamplifiers. As the carrier signal fades to a maximum, or in other wordsincreases in amplitude, the

' plate current of the detector will increase,

which in turn results in a greater voltage drop along the resistance,such as R in the plate circuit; this voltage drop along the resistanceis applied to the grid of the radio frequency amplifier so as to makethe grid more nega tive with increasing plate current and less negative,from a mean Value, for decreasing plate current. An increase in thenegative grid potential applied to the amplifier tube will reduce thegain obtained from that tube. Obviously, the converse of this relationis also effected. Hence, with the amplitude of each signal automaticallymaintained by the above arrangements, within the limits of this con trolmethod, the constancy of the amplitude of the combined signals will bematerially improved.

In Fig. 2 is shown a circuit arrangement similar to that shown in Fig. 1except that the gain regulation of the radio frequency amplifiers isaccomplished through the use of the voltage drop along a singleresistance R- due to the combined current from the detectors D and 1),.This will result in an improvement in the stability of the resultantsignal over that obtained from separate gain regulation of the signalsbefore combination at the output of the detectors. In formerarrangements shown in Fig. 1 the gain regulation must keep each signalat a substantially uni form level or the resultant from the combinationwill depart materially from a mean value, while in the arrangementsshown in Fig. 2 the gain in both circuits is modified simultaneously tomaintain the resultant signal at a mean level.

It is pointedv out that the receiving systems above described arelimited in their ell'ectiveness by the fact that the signals from anyreceiver are changing in phase as well as amplitude, hence, thecombination of two or more signals at the outputs of the detectorcircuits will not be the algebraic sum but the vector sum of the severalsignals.

It is pointed out that the receiving systems of the invention may beutilized in connection with antennae separated in space. The loopantenna might be used at one point and a vertical antenna employed atanother. Likewise, the use of two an tennai at a common point need notbe limited to crossed loops. The loop and vertical antennae may be used,or a vertical and horizontal combination, or the signal from threeantenme at one point may be combined, as would be obtained bv the use oftwo crossed loops and a vertical antenna.

Accordingly, while the invention has been disclosed as embodied incertain specific arrangements, it is pointed out that it may equallywell be embodied in many other and widely varied forms without departingfrom the spirit of the invention as defined by the appended claims.

IV h at is claimed is:

1. A 'adio receiving system comprising two separate circuits eachcomprising an antenna, an amplifier, and a detector, at common receivercircuit, transformer means to conlbine the outputs of said detectors onsaid common receiver circuit, resistances in the plate circuits of saiddetectors, and means to utilize the voltage drop across said resistancesto regulate the gain of said amplifiers.

2. The method of reducing fading of radio signals which comprisesseparately detecting said signals, combining said separately detectedsignals, and utilizing variatimis in a voltage drop in the outputs ofsaid separate steps of detection to automatically regulate the gaintherein.

In testimony whereof, I have signed my name to this specification this23rd day of February 1927.

DE LOSS K. MAl-tllN.

